Furnace



FIG. 1 is a top, front and right side perspective view of a furnace according to our new design;

FIG. 2 is a front elevation view thereof;

FIG. 3 is a right side elevation view thereof;

FIG. 4 is a left side elevation view thereof;

FIG. 5 is a rear elevation view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof.

The broken lines illustrate portions of the furnace that form no part of the claimed design. 

CLAIM The ornamental design for a furnace, as shown and described. 